Conservation is generally
not a dramatic process. It is frequently imperceptible and by its very
nature, usually subtle. Consequently, cleaning can be one of the most
satisfying processes of conservation because its results are immediately
visible, and it appeals to building owners since their investment is readily
seen. However, focusing on the aesthetic benefits of cleaning does risk
overlooking the cause of the soiling and ignoring the history of the building.
Cleaning has become one of the most controversial aspects of conservation,
raising fundamental questions. Is it always necessary or even beneficial?
Are we too ready to clean? Many buildings have been damaged by cleaning
in the past, and even the most appropriate cleaning techniques can be
harmful. Arguably the most beneficial aspects of cleaning are to reveal
the condition of the building where the dirt may have concealed cracks
or structural faults and to slow down deterioration by removing damaging
materials.

Of the various methods
available, nebulous spray cleaning is among the gentlest. This article
looks at some of the many factors to be considered before selecting this
cleaning method as the most appropriate one, as well as giving a very
general account of what it involves.

TYPES
OF SOILING

Dirt
or ‘soiling’ can simply be defined as material which is in the wrong place1.
The question is how to remove this material without causing irreversible
damage to the material which is in the right place, either directly or
by introducing new material. To clean a building or material successfully
we need to start by understanding the nature of the dirt.

Dirt
or soiling may take many forms: airborne particles, gaseous pollutants
and organic aerosols from industrial or vehicular emissions; biological
soiling by algae, fungi, bacteria and lichen; non-biological soiling by
iron staining, paint or graffiti, for example; and the list goes on. In
turn, these may all be affected by water, temperature and wind, and by
the effects of microclimate.

It
may be that the soiling causes stone deterioration or decay, or reduces
the permeability of the substrate; or it may simply appear as an unsightly
surface discoloration. Over time architectural surfaces build up a patina
that is due in part to airborne particles, weathering cycles and the mineralogy
of the stone itself. Unlike surface dirt, the patina does not simply lie
across the surface of the stone but is combined to varying depths within
the masonry, be it stone, brick or terracotta. Although not necessarily
damaging in itself, removing this layer detracts from the historic interest
of the original and may expose a weaker substrate to decay. Another consequence
of removing the build-up of patinas or encrustations is the potential
mobilisation of minerals beneath the stone surface, leading to discolouration.

Consideration
should also be given to potential re-soiling of the stone. Industrial
emissions and environmental factors have changed since many of our buildings
were last cleaned, and it is unlikely that re-soiling will take the same
form.

SELECTING
THE CLEANING METHOD

To
select a cleaning method or even to assess the need for cleaning, it is
important to survey the building first. The aim is to establish the types
of material, their condition, the architectural style, previous treatments
and the nature, cause and pattern of the soiling for each area. All these
criteria must be considered in the context of the building itself, its
history, construction, location and proximity to other buildings etc.

Next,
cleaning trials should be carried out on inconspicuous areas, preferably
using the operator who will be doing the work finally, as skill is just
as important as method. The trial will help to:

further
ensure that the correct method or methods are selected

determine
how clean the surface can become (the ‘level of clean’) without risk
to the fabric

highlight
potential problems.

Trial
areas should be selected on their ability to illustrate as far as possible
the range of soiling types and fabric conditions, to establish levels
of clean which are not just desirable but also achievable, with the least
risk.

Bear
in mind that a uniform surface is rarely achieved without excessive and
highly damaging masonry cleaning. An uneven patchy finish is more likely
as buildings are subjected to a variety of weathering patterns: regularly
rain-washed areas often appear brighter than protected areas, particularly
on limestone buildings: and flat facades may also have uneven soiling
due to apparently similar stones varying in porosity, pore size, capillary
action, or surface texture. The art of cleaning, on aesthetic grounds,
is to find the balance between the extremes. Often it is better to under-clean.

APPROACHES

There
is a multitude of different cleaning methods, which may be wet or dry,
chemical- or water-based, abrasive and nonabrasive, many of which have
a place in conservation. There are positive and negative points to all
methods and there is rarely a single method suitable for all situations.
The least harmful method or combination of methods should be selected
for each case.

NEBULOUS
SPRAY OR INTERMITTENT MIST SPRAY

Low-pressure
water washing is probably the least aggressive form of cleaning. Its application
is particularly useful where water-soluble dirt is present or water-soluble
chemical compounds bind the dirt. Thicker encrustations of soiling which
tend to form in protected areas of a building not regularly washed by
rain may be softened by the water and subsequently mechanically removed.
However, it cannot be used to remove soiling or staining which is insoluble
in water.

Nebulous
spray, also known as intermittent mist spray, is a development of low-pressure
water washing. The aim is to apply the minimum amount of water for the
minimum duration to soften the dirt, thereby enabling its removal by scrubbing
or other relatively gentle treatment. Ordinary low pressure water washing,
by comparison, risks saturating the masonry, causing damage to the wall
by mobilising salts and causing fixings to corrode for example, as well
as damaging other features fixed to the wall such as internal plasterwork,
timber or decorations. It can also lead to dry rot.

Only
once all the investigations have been carried out, questions answered,
options considered and the conclusion drawn that nebulous water spray
cleaning fulfils all the criteria, should cleaning be commenced by those
trained and skilled in the use of this cleaning method and following the
guidelines established during trials.

GENERAL
PROCESS

The
system of nebulous sprays is based on the principle of passing water through
a very fine mesh or filter to create a mist that is then passed through
fine nozzles. The mist spray system can be set up with nozzles at intervals
along the building, concentrating on areas of greater need and reducing
the level where less dirt is present. The level of water may be controlled
electronically or by timers, allowing pulse or intermittent spraying,
to avoid ever having water running down the face of the building. Before
starting, the porosity of the stone can be assessed in order to balance
the amount of water and duration required.

As
the system produces such a fine mist it is important to place the nozzles
close to the building’s surface in order to ensure the water is directed
correctly. Depending on the location and exposure of the elevation it
is frequently necessary to erect a screen to reduce the risk of wind disturbance.

Nebulous
spray systems can be designed to be incredibly flexible, directing the
spray only where needed. Straight or flexible hoses may be employed depending
on the requirements of the surface being treated and the nozzles from
the hose may be grouped or spaced according to the severity of the dirt
or encrustation being treated. Flat surfaces often require less water
than a carved heavily soiled detail, which may require a cluster of nozzles
positioned on an articulated hose to the profile of the carving.

ADVANTAGES

The
most obvious advantages of cleaning with water are that water is cheap,
readily available, safe and environmentally friendly. It is also particularly
effective for cleaning limestone and marble.

The
impact of the mist on the surface is negligible, reducing the risk of
mechanical damage unless the surface is extremely friable. Consequently
the risk of washing away weak pointing material or decaying stone is almost
entirely eliminated.

Encrustations
and dirt are softened progressively, reducing the risk of mechanical damage,
and allowing greater control over removal and permitting more frequent
monitoring of the surfaces. This ensures that the right levels of clean
are achieved and reduces the risk of over cleaning. It also gives greater
opportunity to re-evaluate the method or levels of cleaning than with
many other cleaning methods.

Where
the use of harsher methods of cleaning are unavoidable, prolonged use
may be reduced by first cleaning with the nebulous spray system.

Removing
softened material by brush between spraying cycles may accelerate the
cleaning process and has the added advantage of enabling progress to be
monitored.

A
further advantage is the ability to control the quantity of water used.
Excess run off, which this method avoids, is a particular problem with
traditional water washing methods where weathered wash patterns formed
by rainwater may channel the spray, avoiding adjacent areas of the masonry.
As mist sprays use less water, a more even wash is achieved, avoiding
the weathered wash channels and reducing the probability of saturation
as the stone does not get so wet.

DISADVANTAGES
AND RISKS

Although
the nebulous spray system reduces the risk of saturation enormously,
this problem may still arise as a result of a failure in the timer,
switch or in judging the porosity of the stone which can mean damage
to internal finishes, hidden timber and ferrous fixings.

Water
cleaning methods may exacerbate deterioration when used on badly deteriorated
masonry. The risk of water penetration through defective joints or fractures
is still present with the nebulous spray system, illustrating the importance
of carrying out a thorough survey externally, and continuous monitoring
of the interior as cleaning progresses.

As
with all water treatments, the work should not be carried out when there
is potential for frost damage.

The
network of hoses and bars situated close to the face of the building
can restrict access and make monitoring or brushing down awkward.

Efflorescence
on the surface is possible where water treatments are carried out. Generally
it is possible to estimate the risk of this prior to commencement.

Water
cleaning is less effective on siliceous stones such as granite and sandstone
where the soiling is tightly bound to the silicate surface in insoluble
compounds. Dirt on limestone is generally bound to relatively soluble
chemical compounds.

A
frequent problem with many limestones and some sandstones is brown or
orange staining caused by naturally occurring free iron within the stone
being mobilised and carried to the surface. Consideration must also
be given to the possibility of previous treatments, which may have been
carried out, such as the application of a solution of copperas (ferrous
sulphate) to Portland limestone in the 19th century in order to emulate
the more fashionable Bathstone. Earlier conservation or cleaning treatments
may also have a detrimental effect on the success of water cleaning.

Finally
the set-up and cleaning time required for the nebulous spray is greater
than many other cleaning methods, however, this must be weighed against
the increased control and gentleness of this type of spray.

RECENT
DEVELOPMENTS IN TECHNOLOGY

Time
controllers can be programmed to open a valve for a set period, the length
and frequency of spray being determined by the nature of the material
being treated. Water flow meters are available to measure the quantity
of delivered water and to calculate the output for sprays. The use of
articulated pipe allows greater control over the location of the nozzles.

FUTURE
DEVELOPMENTS

The
employment of moisture switches, which react to differing levels of moisture
in the stone, may negate the need to predetermine the porosity of the
stone.

F Matero et al, 'An approach to the evaluation of cleaning methods
for unglazed architectural terracotta in the USA', Architectural
Ceramics: Their History, Manufacture and Conservation, A joint symposium
of English Heritage and the United Kingdom Institute for Conservation,
22–25 September 1994, James & James, 1996